Tryptophan synthase. The structure, function, and subunit interaction in the multienzyme complex are being investigated. The alpha subuunit contains 2 independently folding domains which can be separated after limited proteolysis. Circular dichroism studies show that the guanidine hydrochloride-induced unfolding of the alpha subunit is stepwise: the smaller C-terminal domain unfolds at low concentrations of denatuant to yield a partially unfolded intermediate; the larger N-terminal domain unfolds at higher concentrations of denaturant. The sites of interaction between the alpha and beta subunits and between the alpha domains are being mapped b chemical modification experiments. An active site arginine in the beta subunit has been modified by phenylglyoxal and is being located in the sequence. The stereochemistry of the sodium borohyhdride reduction of Schiff bases of the coenzyme, pyridoxal phosphate, with amino groups of a substrate and of an active site lysine, has been determined.